Normally, a fluorine-based gas is generated by an electrolytic cell 1 of a fluorine/fluoride gas generator as shown in the schematic view of FIG. 1. As the material of the electrolytic cell 1, Ni, monel metal, and carbon steel, etc., are used. The inside of the electrolytic cell 1 is filled with potassium fluoride-hydrogen fluoride or ammonium fluoride-hydrogen fluoride mixed molten salt as an electrolyte 2. The mixed molten salt to be used as the electrolyte 2 has a melting point higher than the ambient temperature, and the normal electrolytic cell 1 for generating fluorine-based gas has a heating device 12 (temperature adjusting means) such as a heater or a hot water pipe, etc., on its outer peripheral portion. The melting point of the mixed molten salt to be used for the electrolyte is, for example, approximately 70 degrees C. (KF-2HF) or approximately 50 degrees C. (NH4F-2HF)
The electrolytic cell 1 is divided into an anode chamber 3 and a cathode chamber 4 by a partition 16 made of monel metal or the like. By the electrolysis, as a result of applying a voltage between a carbon or nickel (hereinafter, referred to as Ni) anode 51 housed in the anode chamber 3 and an Ni cathode 52 housed in the cathode chamber 4, a fluorine-based gas is generated in the anode chamber 3 side, and hydrogen gas is generated in the cathode chamber 4 side. The generated fluorine-based gas is exhausted from a fluorine-based gas exhaust port 22, and the hydrogen gas generated in the cathode chamber 4 side is exhausted from a hydrogen gas exhaust port 23. By the electrolysis, the electrolysis raw material is reduced. In the case of a potassium fluoride-hydrogen fluoride electrolyte, according to electrolysis, hydrogen fluoride (hereinafter, referred to as HF) is consumed and the electrolyte liquid level lowers. At this time, from a raw material gas supply port 26 extending from the outside of the electrolytic cell 1 1 to the inside of the electrolyte 2 of the cathode chamber, an HF gas as a raw material gas is directly supplied into the electrolyte 2. HF has a boiling point of approximately 20 degrees C., and it is supplied in the form of gas to the gas generator, so that the raw material gas supply pipe 25 must be heated to approximately 35 to 40 degrees C., and it has a temperature adjusting means. Similarly, in the case of an ammonium fluoride-hydrogen fluoride electrolyte, when the liquid level lowers according to electrolysis, HF gas and NH3 gas are directly supplied into the electrolyte 2 from the raw material gas supply pipe 25 extending from the outside of the electrolytic cell 1 into the electrolyte 2 of the cathode chamber and an ammonia (hereinafter, referred to as NH3) gas supply pipe with the same constitution as that of the HF gas supply pipe although this is not shown. The supply of the HF gas and NH3 gas is interlocked with liquid level detection sensors 5 and 6 which monitor the height of the level of the electrolyte 2 so as to maintain a constant liquid level.
As the above-described gas generator, for example, one is disclosed in Patent document 1 listed below.
In the above-described fluorine/fluoride gas generator, when the supply of the raw material gas from the raw material gas supply pipe 25 is stopped due to emergency stop such as a sudden power cut, the raw material gas remaining in the pipe quickly dissolves into the electrolyte 2, so that the inside of the raw material supply pipe 25 leading to the cathode chamber 4 is decompressed. The electrolyte 2 is low in viscosity in a molten state, and it is suctioned to the inside of the raw material gas supply pipe 25 via the raw material gas supply port 26. The heating condition of the heater 24 attached to the raw material gas supply pipe 25 is 35 to 40 degrees C., and this is lower than the melting point of 50 to 70 degrees C. of the electrolyte 2, so that the ingredients of the electrolyte 2 that have entered inside the raw material gas supply pipe 25 are cooled and solidified. The whole raw material gas supply pipe 25 clogged by the solidification of the ingredients of the electrolyte 2 must be replaced, however, this replacement is dangerous, and time and cost are necessary to recover the generator.
The melting point of potassium fluoride-hydrogen fluoride or ammonium fluoride-hydrogen fluoride mixed molten salt fluctuates according to the relative proportions of the ingredients. Particularly, mixed molten salt for an electrolyte to be generally used for generating fluorine is KF-2HF, and its melting point is 70 degrees C. In detail, the ratio of HF to KF in the electrolyte is controlled in the range of 1.9 to 2.3. Herein, at an HF concentration lower than a lower limit of KF-1.9HF, the melting point of the electrolyte suddenly rises and exceeds 100 degrees C. When the melting point is over the control capability of the gas generator, the molten state of the electrolyte cannot be maintained, and as a result, electrolysis cannot be performed, and the gas generator fails. At an HF concentration over an upper limit of KF-2.3HF, the melting point of the electrolyte lowers, however, the carbon-made anode collapses, and if HF increases, the gas generator corrodes. In both of these cases, stable gas supply cannot be performed. In consideration of these facts, to operate the gas generator without problems, stable supply of the raw material gas to the electrolyte must be continued.
As a method for solving the problem of clogging of the raw material gas supply pipe with the electrolyte in Patent document 1, for example, there is proposed a method described in Patent document 2 listed below. In detail, as shown in FIG. 2, the raw material gas supply pipe 25 is provided with a nitrogen gas supply pipe 40 and various members for controlling the flow in the nitrogen gas supply pipe 40. First, nitrogen to be supplied to the nitrogen supply pipe 40 is adjusted in pressure by a decompression valve 46, and temporarily stored in a nitrogen tank 44 through an automatic valve 45. Nitrogen stored in the nitrogen tank 44 is adjusted in pressure again by a decompression valve 43 and adjusted in flow rate by a flowmeter 42 in the nitrogen supply pipe 40, and then supplied to the raw material gas supply pipe 25 through an automatic valve 41. As for operations in detail, first, when liquid level detection sensors 5 and 6 which are installed inside the electrolytic cell 1 and monitor the liquid level of the electrolyte 2 detect a liquid level lower than a reference, an automatic valve 81 opens and supplies the raw material gas to the raw material gas supply pipe 25, and at this time, the automatic valve 41 does not open and nitrogen gas does not flow. When the liquid level detection sensors 5 and 6 which are installed inside the electrolytic cell 1 and monitor the liquid level of the electrolyte 2 detect a liquid level rise to the reference, the automatic valve 81 closes and the raw material gas inside the raw material gas supply pipe 25 is not supplied. At this time, when the raw material gas remains inside the raw material gas supply pipe 25, it quickly dissolves into the electrolyte 2, so that the inside of the raw material gas supply pipe 25 leading to the cathode chamber 4 is decompressed. The electrolyte 2 is low in viscosity in a molten state, and it is suctioned to the inside of the raw material gas supply pipe 25 via the raw material gas supply port 26. The heating condition of the heater 24 attached to the raw material gas supply pipe 25 is 35 to 40 degrees C., and this is lower than the melting point of 50 to 70 degrees C. of the electrolyte 2, so that a part of the electrolyte 2 that has entered inside the raw material gas supply pipe 25 is cooled and solidified. To prevent this suctioning of the electrolyte 2, the automatic valve 41 is opened and nitrogen gas is supplied into the raw material gas supply pipe 25 to wash out all raw material gas remaining inside the raw material gas supply pipe 25 into the electrolyte 2, whereby the inside of the raw material gas supply pipe 25 is cleaned.
Patent document 1: Published Japanese Translations of PCT International Publication for Patent Application No. 9-505853
Patent document 2: Japanese Patent Publication No. 3527735